L-carnitine (3-hydroxy-4-trimethylaminobutyrate) is the exclusively physiologically active form of carnitine. It is widely known in the animal kingdom and has also been identified in plants. D-carnitine has never been identified in animal tissues. L-carnitine transports long-chain, activated fatty acids through the inner mitochondrial membrane into the mitochondrial matrix, where the .beta.-oxidation of the fatty acids takes place. Shorter-chain fatty acids can freely pass the inner mitochondrial membrane. A deficiency of carnitine therefore has an adverse influence on the oxidation of long-chain fatty acids. Carnitine deficiency phenomena are treated by substitution therapy with L-carnitine. Since the necessary amounts of L-carnitine has not always been available, the racemic DL-carnitine has often been administered. DL-carnitine has been used in Europe as appetite stimulant, as additive for sports food and for treating cardiac diseases and obesity. Recently, however, there has been a tendency to administer only L-carnitine since it was determined that D-carnitine is a competitive inhibitor of carnitine-bound enzymes and can consequently cause undesired side effects.
Since the discovery of L-carnitine in muscle, L-carnitine has been obtained by expensive isolation and purification methods from animal tissue. In the fifties, multistage chemical syntheses were developed. However, these methods produce DL-carnitine. In order to isolate the L- isomer, it has been necessary to use methods based on racemate splitting by means of fractional crystallization, using optically active acids. Numerous biochemical methods for producing L-carnitine have also been described: For example, the hydroxylation of .gamma.-butyrobetaine, the reduction of 3-dehydrocarnitine, the conversion of crotonobetaine, the hydrolysis of DL-carnitine nitrile. All these methods have disadvantages from the standpoint of industrial application, since either the initial materials are expensive, the enzymes used are unstable or have insufficient stereoselectivity or activity or the use of expensive coenzymes is necessary.